Underwater reflector of sound waves

ABSTRACT

An underwater reflector of sound waves adapted for use at great depths and comprising a hermetic enclosure constituted of a flexible material capable of undergoing deformation, a stack of mesh members being mounted between two rigid plates within the enclosure. Each mesh member is constituted by crossed filaments and preferably interlaced metallic filaments. One utilization is the forming of reflector surfaces or screens of acoustic antennas for emitter or receiver purposes.

United States atent Cluzel Aug. 26, 1975 [54] UNDERWATER REFLECTOR OF SOUND 2,811,216 10/1957 Harris 181/33 R WAVES 2,884,084 4/1959 Sussman 181/175 3,756,345 9/1973 D'Amico et al. 181/33 G [75] Inventor: Philippe Henri Maurice Cluzel, SIX

Fours 1a Place, France Primary Examiner-Joseph W. l-lartary Assigneei Francaise, Pans, Fraflcc Assistant Examiner lohn F. Gonzales I [22] Filed: Aug. 5, 1974 Attorney, Agent, or Firm-Waters, Schwartz & Nissen [21] Appl. No.2 494,909

[57] ABSTRACT Foreign Application Priority Data An underwater reflector of sound waves adapted for Aug. 16, 1973 France .1 73.29805 use at great depths and comprising a hermetic enclosure constituted of a flexible material capable of un- [52] S Cl 81/1 5; 1 /33 340/8 FT dergoing deformation, a stack of mesh members being [51] Int. Cl. G10K 11/00 mounted between two rigid plates within the enclo- [58] Field Of r h 340/3 8 8 sure. Each mesh member is constituted by crossed fil- 181/400, 402, .5; 343/18 B; 181/33 E, 33 G, aments and preferably interlaced metallic filaments. 33 GA, 33 GD, 33 GE, 33 R, 175 One utilization is the forming of reflector surfaces or screens of acoustic antennas for emitter or receiver [56] References Cited purposes.

UNITED STATES PATENTS 8 Cl 2 D F 1974.951 9/1934 Doorcntz 181/33 0 UX rawmg gums PATENTED AUG2 6 I975 "I '1 In, Hu lh u [I J n f if i "In Go All UNDERWATER REFLECTOR OF SOUND WAVES BACKGROUND 1. Field of Invention The present invention relates to acoustic sound reflectors adapted for use at great depths of immersion in a body of water.

The invention is particularly concerned with the construction of a submersible acoustic reflector for serving as a receiver and an emitter of acoustic waves.

2. Prior Art In a number of devices, it is necessary to utilize walls which reflect sound waves. For example, in emitter or receiver antennas, which are composed of electroacoustic transducers mounted side by side on a rigid support structure, it is necessary to utilize a reflecting screen between the transducers and the support structure.

It is known that the surface of separation between two materials having very different acoustic impedences constitutes a good acoustic reflector.

The reflectors presently utilized underwater in submarines or the like, are generally based on this property.

Water has a relatively high acoustic impedence and rigid materials, notably metals, have an impedence which is, at a maximum, three times that of water.

In contrast, light materials, such as a gas, cork, or cellular material, have an impedence much lower than water and have therefore been utilized for their great impedence difference with water for submerged reflectors. Unfortunately, under high hydrostatic pressure these light materials are compressed or crushed, their mechanical impedence is lowered and the reflector rapidly loses its efficiency at relatively great depths.

SUMMARY OF THE INVENTION An object of the invention is to provide a reflector adapted for use at great depths of immersion of about several hundreds of meters and whose efficiency is substantially independent of pressure for the frequency bands currently used in acoustic submarines.

This objection is achieved by means of a reflector formed by a sealed enclosure containing a flexible and deformable material comprising at least one mesh member of crossed filaments placed between two rigid plates.

The term mesh member of crossed filaments is intended to embrace all structure formed of filaments having substantially point-like regions of contact. Perforated sheets, punched sheets, developed metal sheets are not included in this term as these structures form sheets in which the sound waves travel easily; also the contacts between superposed sheets are in the form of surface areas and the sound waves pass easily from one sheet to another. The reflective power of such an assembly of plates is not good. As reflectors such as structures have a certain reflective power beyond IOKH but at lower frequencies the reflective power is very poor.

In contrast, mesh members with crossed filaments have the property that the sound waves are poorly transmitted not only between juxtaposed members but also in the plane of a member because the contact between the filaments is substantially point-wise.

Preferably, there is utilized a member formed of braided filaments maintained together, without bonding, solely by being interlaced as the transmission of sound waves in such a member is very poor. Such a member has a certain thickness and can be deflected by its elastic flexure. By utilizing a plurality of superposed members placed in a common enclosure, there is obtained an assembly of sufficient thickness to provide a high power reflector which resists high pressure with out permanent deformation due to the elastic flexure of the mesh members.

Preferably, the filaments of the members are made of a metal whose acoustic impedence is greater than that of water. There can also be used wire mesh of any other rigid material having a coefficient of elasticity sufficient to resist crushing.

Rigid plates are placed on opposite sides of the assembly of juxtaposed mesh members for the purpose of transmitting hydrostatic pressure from the flexible enclosure to the mesh member in contact therewith, while distributing the pressure between the points of contact with this member. The hardness of these plates is determined as a function of the number of points of contact of the surfaces in contact for resisting the compression stresses.

These plates and the juxtaposed mesh members which are elastically and deformably interposed therebetween form an elastic system having a regular frequency of oscillation which depends on the weight of the plates. The dimensions thereof are determined for this frequency whereas the frequency of vibration in flexure of the plates is situated outside the frequency band to be reflected in order to avoid all resonance phenomena with the acoustic waves. These plates can be, for example, metallic or statified polyester.

The external enclosure has the function of preventing entry of water into the mesh openings of the mesh member. The enclosure is made of deformable material in order to transmit the hydrostatic pressure to the rigid plates placed in contact with its faces, and therefore it can not resist pressure differences. Elastomer material such as natural or artificial rubber or flexible plastic material are suitable to form this enclosure.

To form relatively elongated surfaces of a reflector according to the invention, there can be utilized panels of large surface area containing in the interior a single enclosure.

According to a characteristic feature of the invention, there is preferably utilized juxtaposed modular units, each comprising a sealed enclosure. In this way, if the enclosures of one or a plurality of units are accidentally broken, the other units remain intact and the reflector will only lose a small part of its reflective power. On the other hand, the mounting of a reflector on a non-planar surface is easier if the reflector is formed of juxtaposed elements.

The invention contemplates the production of a submersible reflector of acoustic waves adapted for immersion to great depths.

Such reflector has the advantage of high reflective power with a relatively reduced thickness of the order of 1 to 2 cm. which, in spite of a small weight, remains high even at great depths of tens of meters of water. Another advantage resides in the fact that the reflective power is high over an extended frequency band currently utilized in acoustic submarines, notably at low frequencies.

A reflector according to the invention can be utilized to form acoustic wave projectors or focussing mirrors, or response screens permitting marking a location under water from the echos of sonar waves reflected by said screen.

The invention will next be described with reference to the annexed drawings showing embodiments of the invention given solely by way of example.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view partly broken away and in section ofa reflector unit according to the invention, and

FIG. 2 is a perspective view of an assembly of reflector units to form an acoustic antenna.

DETAILED DESCRIPTION FIG. 1 shows a reflector unit designated by numeral 1 and composed of a plurality ofjuxtaposed members 2a, 2b, 20 formed of crossed metallic filaments sand wiched between two rigid thin plates 3a, 3b, the assembly being placed in an enclosure 4 which is hermetically sealed and deformable.

In the embodiment of FIG. 1, three mesh members are shown, each member being formed of interlaced warp and weft filaments which confer a thickness to the member of about two times the thickness of the filaments. The assembly of members can deform by the flexure of the filaments around the points of contact between the members.

It is well understood that the number of members can be other than three, either being reducible to a single member or increased above three. Experience has shown that when utilizing five superposed members, a good coefficient of reflection is obtained.

The size of the filaments and the mesh openings is selected as a function of the pressure. The number of points of Contact between the members and between the filaments of the same member is substantially smaller as the mesh size increases. The travel of the acoustic waves in the member is therefore especially reduced as the size of the mesh openings increases and the reflective power is greater. However, the flexural stresses are also greater if the mesh size is increased and there is a risk of obtaining permanent deformation at high pressures.

In practice, there has been successfully used a member of interlaced filaments of steel of a diameter of 0.5 to 2 mm with a mesh size opening between 0.5 mm and 5 cm.

The plates 3a and 3b are thin metallic sheets having a thickness of about 1 mm. They can also be a Stratified synthetic resin.

The enclosure 4 is made of natural or artificial rubber.

The member 2 of interlaced filaments can be replaced by a member of crossing filaments which are joined at the crossing points by ties or spot welding; however the reflective power is lowered at high pressures.

FIGS. 2 shows an acoustic antenna serving as an emitter or receiver of the type utilized in acoustic submarines. The antenna is composed of hydrophones 6 which are electro-acoustic transducers for emitters or receivers. The hydrophones are mounted on rigid support 7.

In order to avoid echos and interferences of the waves reflected by the support 7, there is disposed between the hydrophones and the support a reflective surface 8 formed of identical juxtaposed units or modules 8a. Each unit 8a is constituted of a sealed enclosure containing one or more mesh members of crossed filaments interposed between two rigid plates.

In the embodiment of FIG. 2, surface 8 is planar. It could also have a cylindrical, polygonal, or spherical form in order to constitute a focussing mirror or a projector formed with a great number of modules 8a. The units 8a can be planar or curved.

If the enclosure of one or more units 8a is accidentally burst or broken, the other units conserve their reflective power.

As well understood, one skilled in the art can make a number of modifications to the disclosed embodiments without departing from the framework of the invention as defined in the appended claims.

What is claimed is:

1. An underwater reflector of acoustic waves adapted for high hydrostatic pressures comprising a sealed enclosure of flexible, deformable material, at least one mesh element of crossed filaments, and two rigid plates sandwiching the element therebetween to form an assembly, said assembly being hermetically sealed in said enclosure.

2. A reflector as claimed in claim 1 wherein said filaments are interlaced.

3. A reflector as claimed in claim I wherein a plurality of said elements are provided in superposed rela tion.

4. A reflector as claimed in claim 1 wherein said filaments are metallic.

5. A reflector as claimed in claim 1 wherein said rigid plates are made of Stratified synthetic resin.

6. A reflector as claimed in claim 1 wherein a plurality of said enclosures are assembled in juxtaposed relation to form a reflective surface.

7. A reflector as claimed in claim 1 wherein said plates are in contact with said enclosure.

8. A reflector as claimed in claim 1 wherein said filaments are ofa diameter of 0.5 to 2 mm with a mesh size opening between 0.5 mm and 5 cm. 

1. An underwater reflector of acoustic waves adapted for high hydrostatic pressures comprising a sealed enclosure of flexible, deformable material, at least one mesh element of crossed filaments, and two rigid plates sandwiching the element therebetween to form an assembly, said assembly being hermetically sealed in said enclosure.
 2. A reflector as claimed in claim 1 wherein said filaments are interlaced.
 3. A reflector as claimed in claim 1 wherein a plurality of said elements are provided in superposed relation.
 4. A reflector as claimed in claim 1 wherein said filaments are metallic.
 5. A reflector as claimed in claim 1 wherein said rigid plates are made of stratified synthetic resin.
 6. A reflector as claimed in claim 1 wherein a plurality of said enclosures are assembled in juxtaposed relation to form a reflective surface.
 7. A reflector as claimed in claim 1 wherein said plates are in contact with said enclosure.
 8. A reflector as claimed in claim 1 wherein said filaments are of a diameter of 0.5 to 2 mm with a mesh size opening between 0.5 mm and 5 cm. 